Cationic polyene polymerization

Cooper, W., Polyenes, Chap. 8 in The Chemistry of Cationic Polymerization, P. H. Plesch, ed., Pergamon Press, Oxford, 1963. 

A spectacular example of cationic cyclization is the Johnson polyene cyclization, described in Section 10.8.A. Polyenes such as squalene are expected to assume a steroid-like conformation in the lowest energy form (sec. 1.5.E), based on the biogenetic preparation of cholesterol from squalene. In practice, treatment of polyenes with acid led to a very low yield of tri- or tetracyclic products, giving instead significant amounts of polymeric material. Diligent work over many years prevailed, however, and Johnson solved the many problems (as described in sec. 10.8.A) to make this reaction an excellent and efficient synthetic route to di-, tri-, and tetracyclic molecules. One of the later examples of polyene cyclization uses an allyl silane to quench the cyclization process. A Lewis acid was used to initiate the reaction via reaction with the acetal. Treatment of... 

Aoshima, S., and T. Higashimura. 1984. Vinyl ether oligomers with conjugated-polyene and acetal terminals a new chain-transfer mechanism for cationic polymerization of vinyl ethers. Polymer Journal 16(3) 249-258. 

A reaction called the Johnson polyene cyclization (based on the Stork-Eschenmoser hypothesis) converts triene A into the polycyclic molecule B. When first discovered, an initially formed carbocation at one end of the polyene reacted with a nearby alkene to form a ring containing a new cation. This reacted with another nearby alkene, etc. The reaction was plagued by low yields and formation of polymeric material and decomposition products. This transformation required many years to perfect and two improvements were the use of a cyclopentenol unit on the left to initiate the sequence, which became a cyclopentene unit, and an alkyne unit on the right to end the sequence by generating a methyl ketone. Briefly discuss why these two improvements helped the problems inherent to this reaction. 

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